1. Field of the Invention
The present invention relates to a wire bonding method and apparatus and more particularly to a wire bonding method and apparatus which are used when bonding point coordinates are registered and corrected.
2. Prior Art
FIG. 4 shows one of typical semiconductor devices. In this semiconductor device, pellets 1 are mounted on lead frames 2, and pads 1a1 on the pellets 1 are connected to leads 2a1 on the lead frames 2 by means of wires 3a1. FIG. 5 shows the designations of respective pads, leads and wires to which symbols indicating the bonding order have been assigned. In the following description, pads 1a1, 1a2 . . . 1a14 will be referred to in general as 1an, leads 2a1, 2a2 . . . 2a14 will be referred to in general as 2an, and wires 3a1, 3a2 . . . 3a14 will be referred to in general as 3an. In other words, pads 1an and leads 2an are connected by means of wires 3an.
The wire bonding apparatus which connects the wires 3an is constructed as shown in FIG. 6.
A bonding head 8 is mounted on an XY table 7 which is driven in the X and Y directions by an X-axis motor 5 and a Y-axis motor 6. A bonding arm 9 is installed on this bonding head 8 so as to move upward and downward or pivot. The bonding arm 9 is moved upward and downward or caused to pivot by a Z-axis motor (not shown). A bonding tool 10 is fastened to the tip end of the bonding arm 9, and a wire (not shown) is passed through this bonding tool 10. Furthermore, a camera holding part 11 is fastened to the bonding head 8, and a camera 12 is attached to the tip end of the camera holding part 11 so as to be offset from the bonding tool 10. The workpieces 13 to which wires are connected are fed to the bonding position by a frame feeder 14.
In order to perform bonding using the wire bonding apparatus as described above, it is necessary to register the bonding coordinates beforehand. In the registration of the bonding coordinates, a standard pattern which is used for workpiece alignment is first stored in memory, and positions which are used to store this pattern in memory are registered as fixed points. Afterward, the registration of bonding coordinates is performed. Generally, in semiconductor devices of the type shown in FIG. 4, the relative positional relationship of the respective pads 1an on the pellet 1 is fixed. Furthermore, the relative positional relationship of the respective leads 2an on the lead frame 2 is also fixed. However, since the pellet 1 is fastened to the lead frame 2 by adhesion, the positional relationship between the coordinates on the side of the pellet 1 and the coordinates on the side of the lead frame 2 is usually different in each workpiece 13.
Generally, the bonding positions of the respective pads 1an can be calculated by detecting the shift from the regular positions of at least two fixed points on the pellet 1, and the bonding positions of the respective leads 2 is calculated by detecting the shift from the regular positions of at least two fixed points on the lead frame 2. As a result, bonding is performed in the correct positions of the pads 1an and lead frames 2an. In more complicated semiconductor devices, there may be a detection of positional shifts that use a larger number of fixed points.
An example is described below in which two fixed points on the pellet side and two fixed points on the lead frame side are used. A method that is generally used to perform such an alignment detection of fixed points includes storing a standard pattern of fixed points in memory so that regularizing correlation processing is performed. Naturally, however, other types of detection processing may also be used.
In particular, the registration of fixed points on the side of the pellet 1 is accomplished in the following manner:
Cruciform reticle marks which indicate the center of the camera 12 are displayed on the television monitor (called "monitor") which shows the images obtained by the camera 12; a chessman or digital switch which drives the XY table 7 is operated so that the XY table is moved and the camera 12 is positioned above the pellet 1, and the chessman or digital switch is then operated (while the monitor is viewed by the operator) so that the center of the reticle marks on the monitor is aligned with the desired point. Then, a registration switch is pressed, thus causing the coordinates of the XY table 7 to be registered in the bonding point coordinate memory as a first fixed point. At the same time, the standard pattern of this fixed point is registered in the fixed point standard pattern storage memory. By using a similar operation, a separate point on the pellet 1 is registered as a second fixed point. Incidentally, two separated positions at opposite corners are selected as these fixed points. Fixed points on the side of the lead frame 2 are also registered by means of a similar operation.
After the fixed points are registered, the bonding point coordinates are then registered. As in the case described above, this registration of bonding point coordinates is accomplished in the following manner:
The chessman or digital switch is operated so that the XY table 7 is moved, thus causing the camera 12 to be positioned above the pad 1an or lead 2an that is to be bonded. Then, while viewing the monitor, the operator operates the chessman or digital switch so that the center of the reticle marks on the monitor is aligned with the desired position of the pad 1an or lead 2an. Next, the registration switch is pressed so that the coordinates of the XY table 7 are registered in the bonding point coordinate memory as bonding coordinates. This operation is performed for all of the bonding points, so that the respective bonding point coordinates are registered in the bonding point coordinate memory. Generally, the desired bonding positions are set at the centers of the pads 1an and leads 2an.
By thus registering the coordinates of two fixed points on the side of the pellet 1, two fixed points on the side of the lead frame 2 and the respective bonding points, it is possible to detect any shift of the fixed points in subsequent workpieces 13 of the same type that are to be bonded. Accordingly, the bonding point coordinates can be automatically corrected so that wire bonding can be performed automatically. In other words, the camera 12 is moved to points above the regular fixed points that have been registered beforehand so that any positional shift of the actual fixed points from the regular fixed points is detected. Then, the actual bonding point coordinates of the respective bonding points are automatically calculated on the basis of these positional shift detection values, and the bonding tool 10 is guided to these corrected bonding points (i. e., the XY table 7 is moved to the respective bonding point coordinates), so that the bonding work is performed automatically.
Actual bonding work is accomplished by moving the XY table 7 to coordinates obtained by adding the amount of offset between the center of the camera 12 and the center of the bonding tool 10 to the respective bonding point coordinates that have been corrected for the positional shift, so that the bonding tool 10 is brought to the respective bonding points. The wire bonding methods and apparatuses of this type are disclosed in, for instance, Japanese Patent Application Publication (Kokoku) Nos. 57-11498, 57-33852, 57-50059 and 61-6541.
Conventionally, in order to determine the positions of the bonding points after the coordinates of the fixed points and bonding points have been registered, the following method has been employed:
The XY table 7 is moved to the position of the stored bonding point coordinates, after which the pad 1an or lead 2an and the reticle marks shown on the monitor are examined, and a determination is made to see if there is any positional shift in the bonding point.
However, in cases where it is desired to check or correct bonding point coordinates after bonding has been completed or after the workpiece 13 used for coordinate registration has been sent into a subsequent process, it is necessary to use a method in which the positions of the coordinates are checked using a different workpiece 13 of the same type.
More specifically, in such a conventional method, the shift of the fixed points (i.e., at least two fixed points) from the positions of the regular bonding point coordinates is again detected, and the positions of the actual bonding point coordinates of the respective bonding points are calculated on the basis of these detection values. Then, the XY table 7 is moved to these corrected bonding point coordinate positions, and the pad 1an or lead 2an and reticle marks shown on the monitor in each case are examined to see if there has been any positional shift in the bonding point. Accordingly, the alignment of the two fixed points must be performed accurately, and if there is any shift in this alignment, the respective bonding points will appear to be shifted, so that accurate checking or correction is impossible.